Miniaturization is a feature of modern electronic devices. Most miniaturization occurs in chips, which may, for example, be prepared on silicon wafers through various techniques. Chips generally contain densely packed electronic circuits. However, chips, no matter how small or densely packed with circuits need to be connected to other devices to make a complete electronic-based device. For example, chips may need to be connected to other chips, either directly or through a circuit board, or to other electronic components either directly or indirectly. Thus, chips are typically provided with input and output ports, commonly referred to as bond or bonding pads, to allow for wires to be connected or bonded to the ports to make a functioning device.
Typically, the chips are so small and fragile that it is too difficult to manipulate them directly during a manufacturing process. Therefore electronic manufacturers have made use of substrates or lead frames, which generally take the form of a planar substrate onto which the chips are mounted. The substrate includes a number of bond pads (sometimes referred to as bond fingers) which are electrically connected to the chip bond pads by means of bonding wires. These bond fingers are typically located around the outside edge of the substrate in a non-contacting manner (to prevent short circuiting). The bond fingers are routed along the substrate to leads (typically metal or solder ball connections), which may then be electrically connected to other components as needed to complete the electrical device. A chip mounted to a substrate and having bond wires extending between the chip bond pads and the leads is referred to as a “packaged” chip.
Currently, the fabrication of an electrical connection between the leads and the bond pads is accomplished by a micro bonding wire bonder. FIG. 1 shows a typical device enclosed in a casing or housing 30 in which uninsulated micro wire 10 is fed from a wire spool 16 through a bond head 28 having a wire holding capillary 12. The wire 10 wraps around the spool 16, and is pulled off the spool and through an internal core 17 to the bond head to supply the micro wire needed to form the wire bonds. The bond head 28 generally refers to a group of elements which can be lowered onto a surface 14 to make a wire bond and then raised above the surface 14. The capillary 12 is a removable part of the bond head and requires periodic replacement due to wear and tear. To facilitate the spooling of wire through the capillary, wire clamp 18 is used. Either the surface 14 can be moved or the bond head can be moved to permit the bond head to contact a different point on the surface. Thus an uninsulated wire can be stitch bonded at first one point and then another point on the surface to complete a needed electrical connection. Typically the capillary 12 will be mounted on an ultrasonic arm 20, which applies enough vibration and pressure to bond the wire 10 to the surface 14.
In the wire bonding method called ball-wedge bonding, a bond ball 22 (shown in ghost outline) is formed at the end of the wire 10 for this purpose. To form the bond ball 22 a section of the wire 10 is extended past the capillary 12 or bond head in the direction of the surface 14 to a free end 11. Located adjacent to the bond head is an arc discharge wand 24, for releasing an arc 25 of electrical energy at the free end 11 of the wire 10. The arc 25 is generated by an Electronic Flame-Off (EFO) circuit 32 which is electrically connected to the wand 24. The wire 10 is grounded separately from and just beyond the nonconductive capillary at a diverter 34 and/or at the wire clamp 18, shown as grounds 26 and 26′ respectively. The arc, released from the wand 24 therefore jumps to the free end 11 of the wire 10, runs along the wire and out through the ground 26 and/or the ground 26′. To ensure electrical contact with the conductive ground 26, the wire 10 is bent around the ground 26 at the diverter 34 as shown.
The arc 25 has sufficient energy to cause the free end 11 of the wire 10 to melt, and as a liquid, due to surface tension, it naturally forms a drop shape or ball 22. The ball 22 can then be pressed and bonded onto the surface to form a good electrical connection. Having the ball 22 means that more conductive material is present and that the bond is formed across a larger cross-sectional area at the wire/surface bond interface, improving both the quality of the electrical connection and the mechanical strength of the wire/surface bond. Further, the presence of a bonded ball 22 enables the capillary 12 to be close enough to the surface 14 to form the bond, while avoiding direct contact with the surface 14. Further, the utilization of a ball connection as the first bond on the chip pad, allows the bond head to move in any direction to facilitate the second bond on the substrate bond finger. This ‘omni-directional’ bonding is a characteristic of the ball bonding method, enabling flexibility and throughput advantages over the ‘uni-directional’ wedge-wedge bonding method.
The micro wires used in this type of wire bonding are uninsulated. The chip packaging and bonding pad allow the inputs and outputs for the chip to be separated by a gap large enough that the uninsulated micro wires can be used without short circuits occurring. However, the use of uninsulated wires means relatively large gaps between wires and places constraints upon the bond wire pattern to avoid short circuits. The requirement for such large gaps frustrates miniaturization and yields unnecessarily long circuit paths which reduce the speed and efficiency of the assembled device. Recently therefore it has been proposed that insulated wires be used. However, insulated wires cannot be bonded in the usual way. More specifically once the wand discharges its energy into the wire, the excess electricity will pass along the wire until it reaches the remote ground 26. At that point the excess electricity will jump through the insulation to the ground 26 burning a hole in the insulation. This leads to damaged wire which cannot be used. Thus, what is needed is a method of bonding and a bonding device which is suitable for use with insulated micro wire and which preserves the quality of the insulation of the wire remote from the bond locations. A difficulty to overcome is the very restricted space available at the bond head to deal with the ground issue.